This invention relates to a method for mounting a printing plate on an offset rotary press and more particularly, it relates to a method for stably fixing a printing plate having a paper or a synthetic resin film as a support on a printing cylinder provided with a gripping rod which is fitted in a groove. This method is especially suitable for printing machines for printing business forms.
A representative printing plate used for printing on offset rotary presses is the so-called presensitized plate (PS plate) having an aluminum plate as support. However, the steps for making PS plates are complicated and recently, printing plates made by direct plate-making methods such as silver salt type and electrophotographic type have become popular.
In these printing plates, normally papers, both-side resin coated papers and synthetic resin films are used as supports for purpose of cost reduction. There are problems that these printing plates using paper and synthetic resin film are not necessarily suitable for printing by printing presses designed to be used for printing with printing plates using an aluminum plate as a support.
For example, the offset rotary printing press with a printing cylinder having a gripping rod fitted in a groove as shown in FIG. 6 is one designed for printing with a PS plate and a printing plate using an aluminum plate as support can be mounted thereon without any problems, but printing plates using paper or a synthetic resin film as a support cannot be stably fixed on the cylinder.
As shown in FIG. 6, a thin oblong printing plate 2 is fixed on cylinder 1 by winding it round outer surface of the cylinder. In this case, hooked portions 3, 3 are formed at a front edge portion 5 and a rear edge portion 6 of the printing plate 2 by folding at the portion 4 (fold 4) and these hooked portions 3 and 3 are inserted into a holding slit 7 formed in cylinder 1 and wound round cylinder 1 through the gripping rod 8 and fixed thereto.
When the printing plate is mounted on the printing press, hooked portion 3 must be inserted into holding slit 7 of cylinder 1 as mentioned above and hence fold 4 must be previously formed.
In case the support for the printing plate is an aluminum plate, when the aluminum plate is folded at a desired angle by a force more than the elastic limit thereof, the plate is not restored to the original form and retains its folded state at the desired angle. Therefore, this printing plate can be fixed in close contact with the surface of the cylinder by binding with the gripping rod.
When the support is paper or a synthetic film and when the printing plate is folded at a desired angle, this folding angle is not retained due to the force for restoration to the original form and if it is attempted to bind the printing plate by means of the gripping rod, the printing plate slips out of the slit of the gripping rod and cannot be fixed on the printing cylinder.
Hitherto, printing plates using synthetic resin film as a support (referred to as "printing film plate" hereinafter) have been folded by heat or pressure or by a scribing knife.
When heat or pressure is employed, for example, as shown in FIG. 7, a printing film plate 2 is put on copy mold 9 which has the same width as the printing film plate and has tapering face 10 at both ends and both ends of the printing film plate are pressed in the directions of upper arrow a and lower arrow b by pressing means 12 and simultaneously the tapering face 10 is heated by heater 11 to fold the printing film plate. However, when the film plate is folded by this folding method, as shown in FIG. 8, the fold 4 is merely given a kink and sharp fold 4 cannot be formed and only an obtuse edge can be obtained. When printing film plate 2 having such fold 4 is mounted on a printing cylinder, since a convex part is formed at the folded portion, clear printed images cannot be obtained and besides the folding operation requires much time and the folding device is expensive.
Another method of folding with use of a scribing knife will be explained with reference to FIG. 9. According to this method, groove 14 is formed in both end portions of the film plate through the width by use of scribing knife 13 by an operator. However, it is very difficult to produce groove 14 uniform in depth and the plate is often cut off at that portion. Furthermore, depth of the groove at end portion, in other words, side portion is liable to become smaller while that of the center portion is liable to become greater because of non-uniform application of force.
Other methods to fold printing film plate are: One according to which a printing film plate is folded by a folding machine for a PS plate to form portions to be inserted into the holding slit of a printing cylinder or the slit of a gripping rod at both ends of the plate, namely, the hooked portions, a thin iron strip is applied to said hooked portions and these portions are inserted into the holding slit of printing cylinder and the slit of a gripping rod to prevent the printing film plate from slipping out of these slits and another one according to which an elastic thin iron strip is inserted into the slit of a gripping rod with it protruding therefrom together with one end of the printing film plate and another end portion of the printing film plate is inserted into the holding slit of the printing cylinder and is pressed by the protruded portion of said elastic iron strip by rotating the gripping rod thereby to prevent the printing film plate from slipping out of either slit. In both of these methods, the iron strip must be prepared in accordance with the size of the printing film plate and besides these iron strips must be applied to the plate or inserted into slit at each time of mounting the printing plate on printing cylinder. Thus, mounting operation is troublesome and operability is inferior.
A further method is to directly wind a printing plate round a printing cylinder using double-coated adhesive tape. However, the winding and fixing operation is troublesome and besides the printing plate may often be torn off during printing.